A key State Department official said Friday, the United States will block NATO membership or partner relations with Serbia-Montenegro and Croatia until top Balkans war crimes figures are brought to justice. Undersecretary of State Nicolas Burns spoke as he prepared to begin a Balkans trip next week that will focus on the future of Kosovo.

The Bush administration is maintaining a hard-line on the war crimes issue, despite decisions by the European Union earlier this week to begin accession talks with Croatia and talks on an association agreement with Serbia-Montenegro.

At a news briefing, Undersecretary Burns took issue with an assertion by U.N. war crimes tribunal prosecutor Carla del Ponte that the Zagreb government is cooperating fully with efforts to bring indicted former Croat General Ante Gotovina to justice.

Mr. Burns said Croatia has not made adequate progress in the case, and that regardless of what the EU may do, the United States will block any consensus vote in NATO for Croatian membership, as long as General Gotovina remains at large.

The former Croat military figure is accused of overseeing the "ethnic cleansing" of Serbs from Croatia in 1995, and of having had command responsibility for the killing of some 150 ethnic-Serb civilians.

The undersecretary said U.S. relations with the Belgrade government, and with the ethnic-Serb republic in Bosnia-Herzegovina, will be under similar strain, as long as former Bosnian-Serb president Radovan Karadzic and his military chief, Ratko Mladic, remain at large.

Mr. Burns said that, on his last Balkans trip in June, Belgrade officials told him the apprehension of Ratko Mladic was imminent, and would occur before the 10th anniversary (in July) of the 1995 massacre of Bosnian Muslims at Srebrenica, for which he was indicted.

The State Department official said he was not accusing Belgrade leaders of lying, but that the United States is deeply disappointed and incredulous that the war crimes figures cannot be located.

"I'm not saying I was lied to," Mr. Burns said. "I'm just saying that we were given a very clear view that action was imminent. Now, for whatever reason, the arrest of these individuals did not take place, and so, one of the reasons I want to go to Belgrade is to talk to the government about why that has not happened. But also to let them know that, if they want a full relationship with the United States and with NATO, they're going to have to take these actions, and that message is for the Croatian government, as well."

Mr. Burns, the third-ranking State Department official, will begin the mission next week in Brussels, and go on to Sarajevo, the Kosovo capital, Pristina, and then to Belgrade.

He said the trip is the beginning of a new U.S. policy push on the Balkans, and especially the issue of Kosovo's future, now that the United Nations is recommending the start of talks on the region's final status.

Kosovo, still technically a province of Serbia, has been run by the United Nations since 1999, after NATO airstrikes drove Yugoslav security forces from the area, which has an ethnic Albanian majority.

Undersecretary Burns said the controversial issue of Kosovo's future has been frozen for six years, and that the status quo is no longer sustainable.

"The people of the region have a right to know that they have a future and that they can control that future," he said. "And whether or not the future is of continued association with Serbia-Montenegro, or the future is of independence, that is not a decision for the United Nations or the United States or any of the European countries to make. That is a decision for the people to make. But they have to have a negotiating framework. So, the United States fully supports the decision of Secretary-General Kofi Annan to launch these talks in a fairly short order."

Mr. Burns said he hopes the Kosovo status talks will begin well before the end of the year, and that the United States will name a special envoy to help expedite the process.

He also said the Bush administration will host Balkans leaders in Washington next month to commemorate the 10th anniversary of the Dayton accords on the Balkans conflict, and discuss the future of the region he said is critically important to U.S. interests.